This present invention relates to wellbore completion operations and, more particularly, to a downhole completion assembly for sealing and supporting an open hole section of a wellbore.
Oil and gas wells are drilled into the Earth's crust and extend through various subterranean zones before reaching producing oil and/or gas zones of interest. Some of these subterranean zones may contain water and it is often advantageous to prevent the subsurface water from being produced to the surface with the oil/gas. In some cases, it may be desirable to block gas production in an oil zone, or block oil production in a gas zone. Where multiple oil/gas zones are penetrated by the same borehole, it is sometimes required to isolate the several zones, thereby allowing separate and intelligent production control from each zone for most efficient production. In traditionally completed wells, where a casing string is cemented into the wellbore, external packers are commonly used to provide annular seals or barriers between the casing string and the centrally-located production tubing in order to isolate the various zones.
It is increasingly common, however, to employ completion systems in open hole sections of oil and gas wells. In these wells, the casing string is cemented only in the upper portions of the wellbore while the remaining portions of the wellbore remain uncased and generally open (i.e., “open hole”) to the surrounding subterranean formations and zones. Open hole completions are particularly useful in slanted wellbores that have borehole portions that are deviated and run horizontally for thousands of feet through producing and non-producing zones. Some of the zones traversed by the slanted wellbore may be water zones which must be generally isolated from any hydrocarbon-producing zones. Moreover, the various hydrocarbon-producing zones often exhibit different natural pressures and must be intelligently isolated from each other to prevent flow between adjacent zones and to allow efficient production from the low pressure zones.
In open hole completions, annular isolators are often employed along the length of the open wellbore to allow selective production from, or isolation of, the various portions of the producing zones. As a result, the formations penetrated by the wellbore can be intelligently produced, but the wellbore may still be susceptible to collapse or unwanted sand production. To prevent the collapse of the wellbore and sand production, various steps can be undertaken, such as installing gravel packs and/or sand screens. More modern techniques include the use of expandable tubing in conjunction with sand screens. These types of tubular elements may be run into uncased boreholes and expanded once they are in position using, for example, a hydraulic inflation tool, or by pulling or pushing an expansion cone through the tubular members.
In some applications, the expanded tubular elements provide mechanical support to the uncased wellbore, thereby helping to prevent collapse. In other applications, contact between the tubular element and the borehole wall may serve to restrict or prevent annular flow of fluids outside the production tubing. However, in many cases, due to irregularities in the borehole wall or simply unconsolidated formations, expanded tubing and screens will not prevent annular flow in the borehole. For this reason, annular isolators, such as casing packers, are typically needed to stop annular flow. Use of conventional external casing packers for such open hole completions, however, presents a number of problems. They are significantly less reliable than internal casing packers, they may require an additional trip to set a plug for cement diversion into the packer, and they are generally not compatible with expandable completion screens.
Efforts have been made to form annular isolators in open hole completions by placing a rubber sleeve on expandable tubing and screens and then expanding the tubing to press the rubber sleeve into contact with the borehole wall. These efforts have had limited success due primarily to the variable and unknown actual borehole shape and diameter. Moreover, the thickness of the rubber sleeve must be limited since it adds to the overall tubing diameter, which must be small enough to extend through small diameters as it is run into the borehole. The maximum size is also limited to allow the tubing to be expanded in a nominal or even undersized borehole. On the other hand, in washed out or oversized boreholes, normal tubing expansion is not likely to expand the rubber sleeve enough to contact the borehole wall and thereby form a seal. To form an annular seal or isolator in variable sized boreholes, adjustable or variable expansion tools have been used with some success. Nevertheless, it is difficult to achieve significant stress in the rubber with such variable tools and this type of expansion produces an inner surface of the tubing which follows the shape of the borehole and is not of substantially constant diameter.